Modelling the impact of energetic electrons on electron-acoustic instabilities and HF/UHF wave propagation in the ionospheric-like plasmas.

Recent studies reveal that ionospheric irregularities remain poorly understood due to the lack of models and observations capable of explaining the characteristics of the observed wave structures. In this paper, the effect of energetic electrons is investigated as a driver of such irregularities. Using a particle-in-cell (PIC) simulation framework, we model a two-temperature plasma representative of disturbed ionospheric conditions during energetic electron precipitation. The simulations demonstrate that a minority population of hot electrons provides sufficient free energy to excite electron-acoustic instabilities, which evolve nonlinearly to generate broadband electrostatic fluctuations and field-aligned density structures on kilometre- to decametre-scales. Among these, the decametre-scale modes emerge as the strongest, exhibiting the highest spectral power. These irregularities, in turn, strongly influence the propagation of propagating electromagnetic waves. For high-frequency (HF) signals, the interaction produces scattering, spectral broadening, and envelope distortion, while for ultra-high-frequency (UHF) signals, the effects manifest as localized amplitude growth and amplitude modulation. Since the strongest modes are at decameter scales, they strongly distort HF waves, whose wavelengths are comparable. In contrast, UHF waves, with centimeter-scale wavelengths, interact weakly with these low-power irregularities and experience only minor effects. These results establish a direct physical pathway linking energetic electron precipitation to ionospheric irregularities and to the degradation of radio and GNSS signals. The findings provide both a kinetic-level explanation of observed degradation phenomena and a framework for improving space weather models and forecasting capabilities.